The chemical water treatment industry has historically been involved with reducing or inhibiting the inherent scale forming or fouling tendencies of natural waters associated with large industrial cooling water systems. Many of the foulant components found in water systems originate with the incoming supply, but some contaminants enter the system from the local environment or from process contamination.
Fouling is an extremely complex phenomenon. Fouling of a heat transfer surface is defined as the deposition on a surface of any material which increased the resistance to heat transfer. The fouling tendency of a fluid in contact with a heat transfer surface is a function of many variables including the components of the fluid, which in the case of water include, inter alia, crystals, silt, corrosion products, biological growths, process contaminates, etc. Generally, foulant deposits comprise a combination of several of these materials in relationship to, among other things, the geometry of the heat transfer surface, materials of construction, temperature, etc.
If the fouling tendency of a cooling water system can be accurately predicted before a plant is designed and built, significant capital savings might be realized through more accurate heat exchanger specifications. It is a normal practice to design a heat exchanger with increased heat exchanger surface area to overcome losses in performance caused by fouling deposits with such additional surface area often accounting for more than twenty percent of the actual surface area of the heat exchanger. When such design practice is employed with titanium, stainless steel and similar expensive materials of construction, it can be appreciated that capital expenditures might be significantly reduced if data could be developed to anticipate and provide for an anti-foulant protocol.
U.S. Pat. Nos. 4,339,945 ('945), Re. 33,346 (Re. '346), 4,346,587 (Re. '587) and Re. 33,468 (Re. '468), the entire disclosures of which are incorporated by reference, disclose a mobile apparatus for monitoring and evaluating fouling tendencies of fluids, such as fluid in a cooling water system. The mobile apparatus includes a heat transfer test assembly and related conduit and valve assemblies for connection in fluid flow communication to a heat transfer apparatus for in-situ fouling testing of the fluid passing therethrough, and further includes a monitoring and recording apparatus. The heat transfer test assembly includes a heating rod coaxially positioned within a transparent tubular member for controlled heat input. The heating rod includes a tube member surrounding an insulating matrix in which a heating element is embedded. The test assembly further includes thermocouples to measure the wall temperature of the heating member to permit fouling determinations at varying flow rates with simultaneous monitoring and recording thereof together with data, such as corrosion, pH, conductivity, and the like. The fouling tendency of a fluid may be evaluated by the passage of a fluid through the heat transfer test assembly under controlled rates of flow and heat output from the heating element through measurement of temperature drops between the tube member and the fluid to permit a determination of the resistance of the scale formation therefor. The apparatuses covered by the '945, Re. '346, '587 and Re. '468 patents are marketed by Drew Chemical of Ashland Inc. as the P-U-L-S-E(sm) analyzer.
Current cooling water systems commonly employ heat exchangers having tubes with enhanced heat exchange surfaces (internal and external). Heat exchanger tubes with “enhanced” external surfaces often have external fins to promote more efficient heat exchange, particularly where the external surface is exposed to a condensing refrigerant. Heat exchanger tubes with “enhanced” internal surfaces have internal helical flutes similar to rifling in a gun barrel, particularly where the internal surface is exposed to an aqueous cooling medium. Such enhancements, and particularly internal flutes, promote the precipitation of solids from an aqueous stream and provide an ideal environment for the growth of biomass. In fairly short order, the flutes may become fouled with a biomass rich foulant layer to such an extent that most or all of the benefits of the tube enhancement become neutralized.
Internally enhanced tubes have been found to biofoul at significantly faster rates and to a greater degree than smooth bore tubes. Conversely, smooth bore tubes experience inorganic precipitation/crystallization fouling at a faster rate than internally enhanced tubes. The apparatuses and methods disclosed in the '945, Re. '346, '587 and Re. '468 are effective in accurately evaluating fouling tendencies of fluids in systems using smooth heat exchanger tubes. However, enhanced heat exchanger tubes tend to biofoul faster and to a greater degree than testing using the test apparatuses and methods described in the '945, Re '346, '587 and Re '468 patents will indicate.
In view of the above, there remains a need for an improved method for monitoring fouling in aqueous systems using enhanced heat exchanger tubes. Particularly, there is a need for a method that allows for more rapid detection of biofouling in aqueous systems employing enhanced heat exchanger tubes. Additionally, there is a need for a method that allows for direct, rapid detection of fouling of enhanced heat exchanger tubes as well as smooth heat exchanger tubes.